1. Field of the Invention
The present invention is directed to a device for drawing a group of cladded light conducting fibers where the cores consist of a first glass material and the casings or cladding of a second glass material. The device comprises a double crucible having an inner and an outer crucible with the inner crucible serving to receive a melt for the material forming the core and the outer crucible for receiving a melt from the material forming the cladding. The bottom of the inner crucible as well as the bottom of the outer crucible are provided with nozzles which are arranged in nozzle pairs with each nozzle of the inner crucible being on the axis of the respective nozzle on the outer crucible and the device includes a device for gathering the pulled fibers into a bundle and for coating the gathered fibers prior to their being received on a winding drum.
2. Prior Art
In German Printed Offenlegungsschrift No. 26 30 978, a double crucible having a plurality of nozzle pairs is disclosed. In this device, the interval or spacing between the bottoms of the inner and the outer crucibles is several millimeters and the central axes of each of the nozzle pairs have to be exactly aligned. The ratio of the diameter of the nozzle of the inner crucible and the diameter of the nozzle of the outer crucible depends on the ratio of the fiber core diameter to the overall diameter of the light conducting fiber. When drawing light conducting fibers, the absolute value of the cross section of the fiber will change but the ratio of the partial cross section such as the ratio of the light conducting fiber core to the total cross section of the fiber will remain the same.
The double crucible is situated in a protective chamber having walls made of quartz glass. This protective chamber can be evacuated or charged with a protective gas. It is the purpose of the protective chamber to protect the melt in each of the crucibles from impurities.
The crucibles themselves consist of a heat resisting material, which together with the melt in the crucible does not incur any undesirable reactions. Platinum- or platinum-rhodium alloys are suitable materials for the crucibles.
For heating the double crucibles, induction heating is used which heats the outer crucible. The inner crucible and the melt in the inner crucible are heated by means of heat conduction and/or convection. A system of closing pins or mandrels serve for sealing the nozzle pairs and for the commencement of the drawing operation. Thus, prior to commencement of the drawing operation for the fibers, each of the nozzle pairs is closed by a pin which extends into each of the aligned nozzles of the pair and then during the commencement of the drawing operation, the pins are moved downward from the bottom of the double crucible to begin the drawing of the fiber having the core and cladding.
As soon as the system of pins has passed a feed roller system, the glass fibers attached to the pins are separated from the pins by means of a cutting blade. Further advancement of the fibers is carried out now by the roller system.
The freshly drawn light conducting fibers are then brought together into a group or bunch by a centering cone which is subsequently positioned in their path. The output side of the centering cone is connected to a cross head which is supplied with a liquid synthetic material from an extruder so that the fiber group or bundle is surrounded with a protective tubing or sheath. Following the outlet of the cone and cross head, a cooling bath is arranged through which the fiber bundle or group surrounded with the protective tubing or sheath is drawn and then wound on a winding or take up drum.